The present invention relates to a device for adjusting a nominal value of the dimension of an actuator, a longitudinal dimension of which is changeable by application of an external electromagnetic or electrostatic field, the magnitude of the external electromagnetic or electrostatic field being associated with the longitudinal dimension of the actuator. The device has means for generating a signal characterizing the longitudinal dimension of the actuator, a comparison device that compares the signal characterizing the longitudinal dimension with a predetermined nominal value to detect a deviation of the longitudinal dimension from the predetermined nominal value, and a control device that outputs a correcting variable when a difference between the longitudinal dimension of the actuator and the predetermined nominal value is detected by the comparison device, the correcting variable being predetermined in such a manner that the detected difference disappears.
In a device known from U.S. Pat. No. 4,585,978, the change in length resulting from an external magnetic field of a magnetostrictive actuator is determined by the magnetic induction resulting in the actuator. The change in length thus determined in the magnetostrictive actuator is compared with the nominal value of the change in length. On the basis of this comparison, the external magnetic field is changed in such a manner that a resultant difference between the change in length and the nominal value disappears.
A disadvantageous effect in this known device is that the comparison between the change in length and the nominal value can only be carried out with a magnetostrictive actuator.
A device is also known from German Patent Document DE 24 02 85 A1, in which a rod-shaped body is constructed of a number of stacked laminations or discs of piezoelectric material, and a particular change in length of the rod-shaped body is set by applying an electric voltage. Furthermore, it is stated in this German Patent Document that the relationship between the displacement and the voltage is linear so that a control of the change in length is implemented by adjusting the associated electric voltage.
A disadvantageous effect in this known device is that the laminations or discs of piezoelectric material can change their dimension in dependence on the operating conditions of the rod-shaped body. An "offset" occurs, that is to say the zero-point position of the longitudinal dimension of the laminations or discs changes in the sense that the longitudinal dimension of the rod-shaped body becomes smaller. Thus, problems occur in the control arrangement known from the prior art, to the extent that the accurate association of an electric voltage applied to the laminations with a particular dimension is lost.
It is also known from U.S. Pat. No. 3,414,857 to change the magnetic properties of a core located in the interior of a coil by means of the composition of the individual components and thus to bring the inductance of the coil to a defined value. By this means it is intended to compensate for production tolerances which occur, for example, due to inaccuracies in the winding of the coil.
An object of the present invention is to provide a device for adjusting a nominal value of the dimension of an actuator that adjusts as accurately as possible the nominal value of the longitudinal dimension of the actuator, the longitudinal dimension of which can be changed by applying an external electromagnetic or electrostatic field
This and other objects are achieved by the present invention which provides a device for adjusting a nominal value of the dimension of an actuator, a longitudinal dimension of which is changeable by application of an external electromagnetic or electrostatic field, the magnitude of the external electromagnetic or electrostatic field being associated with the longitudinal dimension of the actuator. The device has means for generating a signal characterizing the longitudinal dimension of the actuator, a comparison device that compares the signal characterizing the longitudinal dimension with a predetermined nominal value to detect a deviation of the longitudinal dimension from the predetermined nominal value, and a control device that outputs a correcting variable when a difference between the longitudinal dimension of the actuator and the predetermined nominal value is detected by the comparison device, the correcting variable being predetermined in such a manner that the detected difference disappears. The device includes an actuator, a position sensor element, and a soft-magnetic and highly magnetoelastic layer, which is part of the position sensor and is connected to the actuator in such a manner that a change in length of the actuator produces a change in the mechanical stress in the layer. An inductance is part of the position sensor element, with a change in the mechanical stress in the layer producing a change in the value of the inductance. The device has means for generating a signal characterizing the change in the longitudinal dimension on the basis of the change in inductance, including means for inferring the mechanical stress in the layer from the magnitude of the inductance and means for inferring the longitudinal dimension of the actuator from the mechanical stress in the layer.
The present invention has advantages compared with the known prior art, both when a piezoelectric actuator and when a magnetostrictive actuator is used. For example, influencing variables, besides the "offset" already described, which lead to a displacement in the zero-point position of the longitudinal dimension of the piezoelectric actuator can also be eliminated by applying the device according to the present invention. As an example, temperature changes lead to a change in the longitudinal dimension of a piezoelectric actuator and of a magnetostrictive actuator. In addition, the device according to the present invention can be used both for magnetostrictive and for piezoelectric actuators.
According to an embodiment of the present invention, a magnetoelastic layer is applied to the actuator. If then a change occurs in the length of the actuator, this change in length produces a mechanical stress in the magnetoelastic layer. The mechanical stress in the magnetoelastic layer can be detected, for example, in that this magnetoelastic layer is part of a coil, the inductance of which thus changes due to the mechanical stress in the magnetoelastic layer.